1. Field of the Invention
The present invention relates to a method of forming a conductive pattern for a semiconductor device and more particularly, to a method of forming a conductive pattern for a semiconductor device, employing a selective tungsten chemical vapor deposition process (hereinafter, referred to as "selective WCVD process").
2. Description of the Prior Art
A method of depositing a metallic film in contact holes of a semiconductor device by a selective WCVD process will be described with reference to FIG. 1. An insulating film 4 is formed over an element isolating film 2. A diffused layer 3 formed in the surface of a silicon substrate (hereinafter, referred to as "Si substrate") 1, and holes 5 for contact holes are subsequently formed in the insulating film 4. Then, a tungsten film (hereinafter, referred to as "W film") 6 is deposited in the holes 5 by a selective WCVD process so that the surface of the W film 6 is flush with that of the insulating film 4. Subsequently, n Al-Si alloy film 7 is formed over the insulating film 4 and the W film 6 by a sputtering process, and then the A-Si alloy film 7 is subjected to photolithographic etching for patterning.
This method provides a highly reliable conductive pattern by filling up the contact holes with a metal to prevent disconnection attributable to faulty step coverage.
In forming the holes 5 by etching, the etching duration is extended beyond a time for overetching by 10 to 15% to compensate irregularities in the surface of the substrate. Consequently, the surfaces of portions of the diffused layer 3 corresponding to the holes 5 (contact holes) are etched in the form of recesses as shown in FIG. 2. The selective WCVD process, in general, uses SiH.sub.4 gas to prevent encroachments and worm holes in the Si substrate, and forms the W film through a chemical reaction: 2WF.sub.6 +3SiH.sub.4 .fwdarw.2W+3SiF.sub.4 +6H.sub.2, in which a chemical reaction: 2WF.sub.6 +3Si (Si substrate).fwdarw.2W+3SiF.sub.4 occurs between the WF.sub.6 gas and the Si substrate.
Accordingly, when the portion of the diffused layer 3 corresponding to the hole 5 is recessed, the WF.sub.6 gas encroaches on the interface between the Si substrate 1 and the insulating film 4. The WF.sub.6 gas, in turn, reacts with the surface of the Si substrate facing the interface to form an encroaching W film 8 in the interface before the W film fills up the recess beyond a level corresponding to the interface between the Si substrate 1 and the insulating film 4. The encroaching W film 8 increases leakage current. It is inferred that, during the deposition of the W film 6, the Si substrate contracts to form a gap in the interface allowing the WF.sub.6 gas to penetrate the interface further, so that the encroaching W film 8 grows gradually in the interface.